In the transportation of hydrocarbons, underwater pipelines are laid on the bed of a body of water and, once laid, pipe the hydrocarbons over distances of hundreds of kilometres. Pipelines of this sort are normally constructed on board a laying vessel and launched onto the bed of the body of water.
Pipelines are constructed and launched using various types of laying vessels, each designed to employ a specific laying method, and which are selected according to the depth of the bed. One known type of laying vessel, suitable for relatively shallow beds, has a substantially horizontal assembly line for constructing the pipeline; and a so-called S-lay ramp—so called from the shape assumed by the pipeline between the laying vessel and the bed of the body of water. Another known type of laying vessel, suitable for deep beds, is designed to join pipe sections along a substantially vertical assembly line, and has a so-called J-lay ramp—also so called from the shape assumed by the pipeline between the laying vessel and the bed of the body of water.
In both cases, the pipeline section between the laying vessel and the bed of the body of water is subjected to bending stress. Specifically, the pipeline forms one curve using the J-lay method, and two curves using the S-lay method. At the laying stage and until the pipeline comes into operation, the pipeline is empty, and the external pressure, especially at exceptional depths, is much greater than the pressure inside the pipeline. It should be appreciated that underwater pipelines may be laid at depths of up to 3,000 metres, so sharp curves combined with the difference between the internal and external pressure may result in crushing, failure, and flooding of the pipeline. This is a relatively extremely serious occurrence that results in irreparable damage of at least part of the pipeline, which must be removed and replaced, and involves clearing the flooded pipeline, which is a relatively highly expensive, time-consuming process.
One known way of minimizing such damage is to insert at least one plug inside the pipeline, move the plug along the pipeline, and expand the plug to prevent the pipeline section upstream from the plug from flooding in the event of the pipeline failing.
Various methods of moving the plug along the pipeline are known. For example, in U.S. Pat. No. 4,498,811 and PCT Patent Application No. 03/042594, the plug is drawn by a cable housed inside the pipeline and operated by a winch on board the laying vessel. In another method, known as “pigging”, the plug is pushed along the pipeline by compressed air supplied from a pumping station, such as a pumping station located at one end of the pipeline. PCT Patent Application No. 2007/040403 describes a plug that is moved along the pipeline by the difference in pressure upstream and downstream from the plug.
Another known way of moving the plug along the pipeline is to use a powered vehicle, which, however, requires an independent power source, as described in U.S. Pat. Nos. 3,837,214 and 3,978,678.
Each of the above described plug driving methods has pros and cons in terms of plug positioning precision, operating range, and production and installation cost and complexity.
The powered vehicle solution, in particular, has been found to perform particularly well along relatively very short pipelines not requiring much power range.